Hi Brook100,
Yes usually the rental business does not begin until the first property is let.
Any expenses incurred before the first let must be solely for the rental business and must not be capital expenditure.
Any qualifying pre-letting expenses are treated as incurred on the day the rental business commences which to confirm is usually the day the property is first let.
The guidance can be found here:
PIM2505 - Beginning and end of a rental business: commencement
Thank you.